1、Designation: E 377 96 (Reapproved 2002)Standard Practice forInternal Temperature Measurements in Low-ConductivityMaterials1This standard is issued under the fixed designation E 377; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, t
2、he year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers methods for instrumenting low-conductivity specimens for testing in an environment su
3、bject torapid thermal changes such as produced by rocket motors,atmospheric re-entry, electric-arc plasma heaters, and so forth.Specifically, practices for bare-wire thermocouple instrumen-tation applicable to sheath-type thermocouples are discussed.1.2 The values stated in inch-pound units are to b
4、e regardedas the standard. The metric equivalents of inch-pound unitsmay be approximate.1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices
5、 and determine the applica-bility of regulatory limitations prior to use.2. Significance and Use2.1 Internal temperature measurements are made on bothin-flight vehicles and on-ground test specimens; and, becauseof the importance of the temperature measurements to thedesign of various missile and spa
6、cecraft heat shields, it isessential that care be taken to minimize the sources of error inobtaining these measurements.2.2 Over the past several years, the problems of usingthermocouples to obtain accurate temperature measurements inlow-conductivity specimens have been studied by variouspeople to i
7、solate the sources of error and to establish improvedtemperature measurement techniques. The major sources oferror are listed in this document and recommended solutions tothe problems are given.3. General3.1 Before proceeding to the major sources of error, it isassumed that the reader is familiar wi
8、th basic methods offorming and using thermocouples2, that is (1) electric weldingto form junctions, (2) maintaining cleanliness of junction areaand lead wires, (3) proper selection of thermocouple type andsize, corresponding to both the temperature range to bemeasured and the chemical compatibility
9、with the environ-ment, and (4) proper use of instrumentation for readout ofthermocouple emf.NOTE 1Reader is referred to ASTM MNL 12, and STP 492, as wellas Kinzie, P.A., Thermocouple Temperature Measurement, for neededinformation.3.2 The most important sources of error beyond the abovebasic areas ar
10、e the following:3.2.1 The thermal disturbance produced in the low-conductivity material at the vicinity of the thermocouple sensorhot junction due to the sensor size, configuration, and instal-lation method.3.2.2 Electrical shorting of lead wires due to the electricalconductivity of the virgin or ch
11、arred ablation material, and3.2.3 Thermocouple sensor hot junction location accuracy.4. Thermal Disturbance at Vicinity of ThermocoupleSensor Hot Junction4.1 GeneralIdeally, to measure the true internal tempera-ture of a solid body, it would be desirable not to have anyforeign substance present that
12、 would create a disturbanceaffecting the natural flow of heat in the body. Since it isphysically impossible to exclude the temperature sensor fromthe internal confines of the body, it is necessary that the thermaldisturbance introduced by the sensor be minimized for accuratetemperature measurements.
13、4.2 Thermocouple Junction Bead Diameter:4.2.1 GeneralExcessively large junction beads result inlower than true temperature measurements in low-conductivitymaterials (conductivity of material less than conductivity ofthermocouple wire) because of the heat sink effect of the bead.4.2.2 Recommendations
14、To minimize this effect, the junc-tion bead diameter should be no larger than 1.5 wire diametersfor butt-welded junctions and 2 wire diameters for other typesof welds.4.3 Thermocouple Wire in Isothermal Surface of Hot Junc-tion:4.3.1 GeneralBecause many materials have low thermalconductivity compare
15、d with those of thermocouple assemblies,it has been found that certain methods of installing sensors canproduce significant errors in internal temperature measurement1This practice is under the jurisdiction of ASTM Committee E21 on SpaceSimulation and Applications of Space Technology and is the dire
16、ct responsibility ofSubcommittee E21.08 on Thermal Protection.Current edition approved Oct. 10, 1996. Published December 1996. Originallypublished as E 377 68. Last previous edition E 377 68 (1992).2ANSI MC96.1-1975. Temperature Measurement Thermocouples (SponsorISA).1Copyright ASTM International, 1
17、00 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.(1-4).3Errors of several hundred degrees are possible unlessheat conduction away from the sensor hot junction, by thesensor materials, is minimized. Test results show that athermocouple having a sufficient length of b
18、are wire in theisothermal surface that includes the junction will minimizethese errors.4.3.2 RecommendationsIt is therefore recommended thatthe configuration of the thermocouple sensor be such that theleads perpendicular to the heat flow have a length equivalent toat least 25 wire diameters on both
19、sides of the junction in thesame isothermal surface that includes the hot junction.4.4 Disturbances in Vicinity of Thermocouple Sensor HotJunctions (2-4):4.4.1 GeneralIt is important that a minimum amount ofdisturbance be created in the material around the thermocouplejunction.4.4.2 RecommendationsT
20、he disturbed material removalarea (for placement of the thermocouple junction and leadwires) should be as small as possible. A maximum of No. 36AWG gage (0.127-mm or 0.005-in.) wire should be used forthe thermocouple wire from the junction and along the isother-mal surface which includes the junctio
21、n. Holes drilled forplacement of thermocouple wires should be 3 wire diameters orsmaller. It is recommended also that the difference in thermalconductivity between thermocouple assembly and the sur-rounding material be minimized by: (1) avoiding the use ofrelatively conductive (thermal) insulation (
22、such as ceramic andfiberglass types) around the portion of wire that is located inthe isothermal surface that includes the thermocouple junction,and (2) maintaining good thermal contact with the low-conductivity material by bonding the thermocouple to thespecimen (thus eliminating air pockets) with
23、the same orsimilar compound (such as an epoxy plastic) as that used tomake the specimen.5. Electrical Shorting by Conductive Char Layers5.1 GeneralThe char layer formed by most organic ma-terials becomes highly conductive (electrically) as pyrolysisprogresses. Care should be taken to avoid the possi
24、bility ofelectrical shorting of thermocouple lead wires not protected byproper insulation methods. Studies (1) have shown that short-ing can result in temperature errors of as much as 110 C (200F) in thermocouples which do not employ proper insulation ofthe lead wires.5.2 RecommendationsIt is recomm
25、ended that electricalshorting be avoided by using a ceramic coating or tubingaround the thermocouple lead wires. Two possible configura-tions are shown in Fig. 1 and Fig. 2. Use of either configurationshould provide accurate measurements in low-temperaturegradient fields (3). In that the wire temper
26、ature at the exit of theceramic cover in Fig. 2 may be substantially different from thatin the vicinity of the hot junction (3), the configuration in Fig.1 should be used in high-temperature gradient fields. Care mustbe taken to select an insulation that does not become electri-cally conductive at t
27、he temperatures being measured.6. Thermocouple Sensor Hot Junction Location Accuracy(1,2)6.1 GeneralThe thermocouple junction needs to be accu-rately located to assure reproducibility of data from specimento specimen and for accurate use of temperature data incomputer programs for determining materi
28、al thickness require-ments, etc.6.2 RecommendationsThe actual location of each thermo-couple should be verified by X ray prior to temperaturemeasurement experiments. Care should be taken to correct3The boldface numbers in parentheses refer to the list of references at the end ofthis practice.NOTE 1I
29、f a number of thermocouples in depth are required, drill holes at varying locations on the circumference.NOTE 2Eliminate air pockets in junction plane by filling hole with same or similar compound as that used to make test specimen.NOTE 3This is a schematic representation and is not intended to be a
30、n engineering drawing.FIG. 1 Summary of Recommended Practices for Mounting ThermocouplesSchematic Representation for “One-Piece” CylindricalSpecimenE 3772X-ray measurements for parallax. Thermocouple junction,lead-wire location, and gas pockets are best checked by two Xrays taken in front and side v
31、iew.7. Summary of Recommendations (also summarized inFig. 1 and Fig. 2)7.1 Thermocouple Junction Bead DiameterMake beadsno larger than 1.5 wire diameters for butt-welded junctions and2 wire diameters for other types of welds.7.2 Thermocouple Lead Wire in Isothermal Surface thatIncludes the JunctionU
32、se a length of wire at least 25 wirediameters on both sides of the junction.7.3 Thermocouple Wire Diameter and Holes for WiresUse a maximum of No. 36 AWG gage wire and holes as smallas possible but no larger than 3 wire diameters.7.4 Thermal Conductance of Thermocouple AssemblyAvoid the use of relat
33、ively conductive insulation around wirein the isothermal surface that includes the junction. Bondthermocouple to the material with same or similar compound.7.5 Electric Shorting by CharUse a ceramic coating ortubing around the thermocouple lead wires.7.6 Thermocouple Sensor Hot Junction LocationAccu
34、racyLocate by X rays taken in front and side view.8. Keywords8.1 internal temperature; low-conductivity; thermocoupleREFERENCES(1) Dow, M. B., “Comparison of Measurements of Internal Temperaturesin Ablation Materials by Various Thermocouple Configurations.”NASA Technical Note D-2165, November, 1964.
35、(2) Moen, W. K., “Significance of Errors in High Temperature Measure-ment,” Society of Automotive Engineers, Inc., Paper No. 750F,presented at National Aeronautic and Space Engineering and Manu-facturing Meeting, Los Angeles, Calif., Sept. 23 to 27, 1963.(3) Beck, J. V., “Thermocouple Temperature Di
36、sturbances in Low Con-ductivity Materials,” Transactions, TASMA, ASME, Journal of HeatTransfer, Series C, Vol 84, 1962, pp. 124132.(4) Beck, J., V., “Study of Thermal Discontinuities and AssociatedTemperature Disturbances in a Solid Subject to a Surface Heat Flux,”Part IIIEffect of Sensors in Low Conductivity Material UponTemperature Distribution and Its Measurement, Technical ReportRAD-TR-9(7)-59-26. Contract Nos. AF 04(647)-305 and AF 04(647)-258.(5) Jakob, M., Heat Transfer, Vol II, John Wiley or through the ASTM website(www.astm.org).E 3774
